Ethylene (co)polymerization typically operates at a temperature that is close to the softening temperature of the resultant (co)polymer. Insufficient heat removal can easily lead to temperature exceeding the softening temperature and cause (co)polymer agglomeration that may disrupt production continuity.
In a gas phase polymerization process, the polymerization reactor is cooled by the circulating monomer gasses to maintain a steady operating temperature. However, if the temperature of a growing resin particle approaches the sticking/melting point of the resin, resin sheeting on the reactor walls may occur. Growing resin particles are especially susceptible to overheating if they accumulate at the reactor walls, thereby losing heat-transfer with the circulating monomer gasses, and remaining in close contact with respect to each other. In such instances, particle-particle fusion may occur, followed by reactor sheeting, which, in turn, could cause reactor shutdown.
Catalyst systems to minimize or prevent reactor sheeting have been developed for use in propylene polymerization reactions. Such catalyst systems possess mitigating chemical features, which reduce polymerization rate when the temperature becomes excessive. A known system uses two or more reagents external to the catalyst composition, one or more Selectivity Control Agent (SCA), or one or more Activity Limiting Agent (ALA), or one or more Self-limiting Agents (SLA), to slow or deactivate the polymerization reaction. The combination of SCAs and SLAs have been used successfully in propylene polymerization and co-polymerization reactions; for example, the mixtures described U.S. Pat. Nos. 7,678,868, 7,381,779, 7,491,670, and US20110152067. The operating temperature for PP polymerization is 65 to 80° C., and the melting point of the resin is about 165° C., giving an 85 to 100° C. temperature span in which an SLA may operate. Generally, polypropylene SLAs substantially shut down the polymerization active site when the temperature of the active site reaches about 90° C. Such polypropylene catalysts may contain an ester or diether compounds as internal donor, which can further suppress catalyst activities at elevated temperatures.
SLAs or the combination of SLAs and SCAs, however, have not previously been used successfully in ethylene polymerization or copolymerization reactions in commercial scales. For a PE process, the polymerization temperatures are about 80 to 112° C., while the melting point of the PE resin produced is about 115 to 135° C. and the sticking temperature, i.e. the temperature at which granular particles begin to adhere to each other, is about 100 to 125° C. Thus, for such PE polymerization reactions, there is only a 15 to 25° C. temperature span in which SLAs may function.
The currently available catalyst systems fail to address such heat removal concerns in PE polymerization systems. Therefore, there is a need for a catalyst system having an effective mechanism that substantially reduces catalyst activity within a narrow temperature range and therefore reducing heat generation when the temperature in various parts of the reactor system approaches (co)polymer softening temperature to prevent agglomeration formation and minimizing production disruptions.